1. Field of the Invention
The present invention generally relates to a cellular communication system used to off-load telephone calls from a main cellular system, and, more particularly, to a cellular system providing telephone service for a mobile telephone in a cell which is located in an area geographically inside a larger cell of a main cellular system requiring a high concentration of cellular service.
2. Description of the Related Art
Cellular mobile telephones (mobile telephones) are typically employed in automobiles, ships or the like, and thus, are transportable with the user into various different geographic zones. In each of the different geographic zones, one or more mobile telephone switching offices (MTSO) are provided to complete call connections. Once connected to the mobile telephone switching office, the mobile telephone may then be connected to another mobile telephone within a specific zone or, through land-based networks such as a public switching telephone network (PSTN), to a land-based telephone or to another cellular mobile telephone in a different zone.
Cellular mobile telephone systems (cellular systems) service specific geographic areas, or zones, each of which is typically divided into a plurality of cells. Each cell includes a stationary transmitter receiver (transceiver) station coupled to transmit and receive antennas, which is used to establish radio communication connections with mobile telephones physically located within the associated cell. The stationary transmitter receiver stations, in turn, are connected to a mobile telephone switching office, associated with the zone where the stationary transmitter receiver stations are located, which switches the call in an appropriate manner. Each cell within a specific zone has a number of frequencies assigned to it to establish radio communications with a mobile telephone. The frequencies are divided into control channels, paging channels and voice channels. The paging and control channels are used for the mutual identification between the mobile telephone and the cellular system providing the cellular mobile telephone service. The location or frequency of the control channels contained in the frequency range assigned to each cell identifies the type of cellular system which is being used. Typically, one set or range of frequencies is called an "A" cellular system and a second set of frequencies is called a "B" cellular system. Generally, a specific geographic area contains one of each type of cellular system (i.e., an A and a B cellular system) thereby to provide alternative cellular service to that area.
A mobile telephone must identify itself to the cellular system provider before service for the mobile telephone is established. The identification process begins using the control channels. When first switched on, the mobile telephone scans through the control channels and measures the signal strength of each channel. It will then tune to the strongest control channel and lock on to the overhead message stream. When the mobile makes an access attempt, the reverse setup channel access message contains various data which permits the cellular system to identify the mobile telephone and thereby determine whether the mobile telephone should be provided with cellular service. A detailed description of the interface between the mobile telephone and the land-based transceiver station may be found in the Federal Register, Vol. 46, No. 98, Thursday, May 21, 1981--app. D, pages 27680-27706.
In a typical cellular telephone system, as a mobile unit travels along a path that passes from one cell to another, a handoff occurs. The handoff action is controlled by the mobile telephone switching office which monitors the signal strength received from the mobile telephone. The handoff command is typically generated when the signal received from the mobile telephone falls below a preselected signal strength, thus indicating that the mobile telephone is at the cell boundary of one cell and requires a transfer of the cellular service to an adjacent cell which is able to receive a signal from the mobile above the preselected signal strength.
As a mobile telephone passes from one cell to another cell, the handoff command instructs the new cell which the mobile telephone is entering, to begin transmitting at a frequency which is different from the frequency which was transmitted by the cell from which the mobile telephone is exiting. This procedure is followed as the mobile telephone passes into each next successive, adjacent cell. The assigned frequencies of each adjacent cell are different, and such assigned frequencies are not repeated except for cells that are far enough away from each other so that no interference problems will occur.
One example of a stationary transmitter receiver station, also referred to as a cell site, is the AT&T series I and II cell sites (model 1 and model 2 architecture) used in the family of AUTOPLEX.TM. cellular telecommunications systems which are commercially available from the American Telephone and Telegraph Company of New York, N.Y.
Because of increased demands for more radio channels or frequencies resulting from an increased number of cellular customers, cellular system providers often desire to expand their networks to serve an increasing number of customers within a geographic location.
Various solutions have been proposed to increase the capacity of cellular systems and thereby to meet the foregoing need. For example, a cell may be split into four smaller cells, each with a radius of half the radius of the original cell permitting cellular service to be increased four fold. Naturally, the smaller the cell, the greater the number of handoffs are required in a cellular telephone system for a given capacity. These smaller cell sites are typically known as microcell sites which operate functionally, similarly to a traditional macrocellular or main cellular system but only provide cellular service for a small geographic area. The microcell sites are typically directly connected to the main cell site using coaxial transmission lines, microwave links or an optical fiber cable network which interconnects the main cell site with associated microcell sites using a nonstandard protocol. See, for example, U.S. Pat. No. 5,067,173 citing additional references.
While cells may be subdivided into smaller cells to provide cellular service for an increasing number of customers, shrinking cell cites creates additional considerations and problems. For example, the rate at which mobile telephones move through the cell and the non-uniformity of the electromagnetic field generated in the cell affect the performance of a microcell system. Both factors relate to the time required to determine the relative location of the mobile telephone and to process a handoff for the mobile telephone from the stationary transmitter receiver station of one cell where the mobile telephone is currently located but is preparing to exit, to the stationary transmitter receiver station of another cell, where the mobile telephone is entering. If a handoff is required, one or more candidate cells must be queried for their idle channel status and for a verification of the mobile telephone's signal strength in that candidate cell. The processing of the decision, status, and verification usually requires the intervention of higher level system control functions in addition to the control function in the serving and candidate cells. In addition, the mobile telephone must be instructed to tune to a frequency available in the candidate cell and verification of its presence after the handoff must be made by the candidate cell. Thus, a significant amount of time is used for handoff processing.
In addition, since microcell systems are usually directly connected to a main cellular system via a nonstandard protocol, it is impossible to integrate or include a generic microcell system to off-load cellular service from the main cellular system since microcell systems are manufactured with a proprietary nonstandard protocol. This inability of competing products to provide cellular service results in microcellular systems being expensive and difficult to integrate with other cellular systems.
Thus, it is desirable to provide for easily and inexpensively off-loading cellular service from a main cellular system using a microcellular system. Specifically, it is desirable to enable establishing a microcellular system in a small area, requiring a high concentration of cellular service, of a main cellular system to thereby off-load a significant number of customers from the main cellular system. For example, a large office building or shopping mall may require a significant amount of cellular service which could be easily provided using a microcellular system since mobile telephone users typically establish cellular service while walking from office to office in a building or from store to store in a shopping mall. Thus, mobile telephones in these small areas are less likely to rapidly move across large geographic areas, making use of a microcellular system ideal.
In addition, there is currently no method of off-loading cellular service from a main cellular system to another or off-load cellular system without reconfiguring the main cellular system to ignore the cellular service in the off-load cellular system which is providing cellular service in the same area where the main cellular system had previously provided the cellular service. It is, therefore, also desirable to provide this additional or off-load cellular service for customers in a high concentration area of the main cellular system without reconfiguring the main cellular system, thereby providing off-load cellular service which is transparent to the main cellular system.
Further, it is desirable to off-load cellular service from a main cellular system using, for example, an off-load or microcellular system to provide cellular service in an area of high concentration without requiring the microcellular system to interface with the mobile telephone switching office (MTSO). The reason that it is undesirable for the off-load cellular system to be required to interface with the mobile telephone switching office is that no standard interface has been developed which allows the off-load cellular system to be easily integrated with the mobile telephone switching office without the off-load cellular system being required to implement the specific interface requirements of the mobile telephone switching office. Since various vendors provide mobile telephone switching offices for cellular service using different interface protocols, the off-load cellular system would be required to know the interface protocol used by each cellular system. Thus, the off-load cellular system would be complex and not easily integratable into an already existing cellular system.
Finally, it is desirable that a mobile telephone, which is being serviced by the off-load cellular system be able to establish telephone service with telephone equipment which is located outside the off-load cellular system. Thus, even though the off-load cellular system is transparent to the main cellular system, the off-load cellular system must still be able to provide telephone service with telephone equipment located outside the serving area of the off-load cellular system.